This invention relates to an optimal surface acoustic wave orientation on single crystal lanthanum gallium silicate or La.sub.3 Ga.sub.5 Si O.sub.14, commonly known as langasite (LSG). SAW devices are currently used as bandpass filters, resonators, delay lines, convolvers, etc. in a broad range of RF and IF applications such as wireless, cellular communication and cable TV. Three commonly used substrates are lithium niobate, ST-Quartz, and lithium tantalate. There are several material properties that determine the usefulness of any particular crystal and, in particular, orientation of a particular crystal. These properties include: 1) SAW velocity, Vs; 2) the SAW piezoelectric coupling coefficient, k.sup.2 ; 3) the power flow angle, PFA; 4) the diffraction or beam spreading coefficient, .gamma. (gamma); and 5) the temperature coefficient of delay, TCD. It has not been possible to find an orientation in any existing substrate which optimizes these properties at the same time; so the choice of substrate and orientation depends upon what is important for the application, and almost always involves a compromise between the SAW material properties. A high velocity is desirable for high frequency devices, because the device geometry patterns are larger and, therefore, the devices are easier to fabricate. At low frequencies, a low velocity is usually desirable because the device size is smaller, resulting in lower devices and packaging costs. Thus, there is no universally optimum velocity. For moderate to wide bandwidth devices, a high value of k.sup.2 is desirable because it allows lower insertion loss. From the point of view of k.sup.2, lithium niobate is best, quartz is worst, and lithium tantalate is in between. For most devices, and in particular narrow band devices, TCD should be as low as possible and ideally zero. From this point of view, ST-Quartz is best, lithium niobate is worst, and lithium tantalate is in between (just the opposite ranking as for k.sup.2). The optimum value of .gamma. is -1, which results in minimum beam spreading. From this point of view, YZ lithium niobate is now ideal, ST-Quartz is worst, and lithium tantalate is in between. The PFA should be zero, and this is the case for most of the commonly used SAW substrates, with an exception being 112.degree. lithium tantalate, which has a PFA of 1.55.degree.. For the most narrow band applications, ST-Quartz is a quite acceptable choice; and for the very wide band applications where temperature stability is not so important (e.g., a device can be held at constant temperatures), lithium niobate is usually quite acceptable. What is needed is a substrate orientation that offers the temperature stability of ST-Quartz but with higher k.sup.2 and at the same time low or zero beam steering (PFA) and diffraction (.gamma.=-1). It is an object of the present invention to provide a substrate that meets these conditions.
Earlier research looked at using langasite to achieve a favorable trade-off in the material properties that determine the coupling, PFA, diffraction, and TCD. The choice of Euler angles were as follows: .phi.=90.degree., .theta.=10.degree., and .psi.=0.degree.. The performance parameters of this orientation were quite poor. The TCD is 12 ppm/.degree. C. (ST-Quartz is near zero), the coupling k.sup.2 =0.26% (better than ST-Quartz, which is 0.11%), PFA=-5.7.degree. (ST-Quartz is zero), and diffraction coefficient .gamma.=-2.86 (also worse than ST-Quartz). Only the coupling is better than ST-Quartz.
A different orientation of langasite has been formed which has far superior properties to both ST-Quartz and the earlier orientation of langasite.